There has been a long standing interest in providing a simplified construction kit for interchangeably building either horizontally or vertically sliding windows. Various constructions have been proposed. Examples of such window constructions are described in the following patents:
U.S. Pat. No. 3,859,754--Budich PA1 U.S. Pat. No. 4,087,941--Wolf PA1 U.S. Pat. No. 4,351,131--Kubik PA1 Canadian Patent 1,138,716--Budich
The various sliding closure constructions disclosed in these patents suffer from certain drawbacks which have largely prevented their commercial acceptance. One common disadvantage of the window constructions of the type disclosed in the patents listed above is that their profile design is not adequately rigid to support a heavy window. As a result, the profiles often require reinforcement in the form of metal box beams and the like. A further disadvantage is that the window assemblies described in the prior art require the addition of wheel assemblies to the sashes of the horizontally sliding window, necessitating extruded track profiles which must be added to the basic assembly. The addition of such track profiles requires machining and fitting of parts which is both time consuming and expensive. Furthermore, such track assemblies often protrude laterally over the edges of the main frame, which distracts from the aesthetic qualities of the window and contributes a structural weak point which may be damaged during handling and/or transport of the window assembly. An additional drawback of the window assemblies of the known prior art is that the sill structure is generally flat and thereby fails to provide adequate surface for efficient weather stripping. This makes those window assemblies vulnerable to the intrusion of air and water and thereby detracts from their desirability.